1. Field of the Invention
This invention relates generally to optical systems for producing coherent, vacuum ultraviolet (VUV) radiation. More particularly, it relates to approaches for generating coherent VUV radiation in the vicinity of the molecular fluorine line (157.63 nm) or the Lyman-xcex1 hydrogen line (121.57 nm) using efficient, phase-matched, two-photon-degenerate four-wave mixing.
2. Description of the Related Art
As technology progresses, there is an increasing demand for ever more powerful integrated circuits or, equivalently, a demand to include ever more circuitry into silicon chips that form integrated circuits. The result is that the circuits are reduced to ever smaller dimensions, requiring that ever finer features must be patterned during the manufacturing process. In order to meet this demand, the microlithography tools which are used to pattern such fine features have also been required to operate at ever shorter wavelengths, with the most recent generation of tools moving toward lasers that produce radiation beams with wavelengths in the ultraviolet region of the electromagnetic spectrum. Specifically, the argon fluoride (ArF) laser is commonly used to produce radiation at a wavelength of 193 nm, representing the currently available technologies. The favored candidate for next generation lithography is the molecular fluorine (F2) laser, which produces radiation in the vacuum ultraviolet (VUV) region of the electromagnetic spectrum, specifically at 157.63094 nm. A possible successor to the F2 laser is a VUV source which operates at the Lyman-xcex1 wavelength of atomic hydrogen (or its isotopes deuterium and tritium), approximately at 121.57 nm.
The technological challenges in transitioning from current to next generation lithography are many. The optical designs for state-of-the-art stepper platforms are complex and require the precise testing and alignment of many large focusing optics. High-energy photons cause significant problems in impure optics, and so the raw materials used for the production of the optics themselves must be of high purity. Dielectric anti-reflection and high-reflection coatings typically are optimized for the operating wavelength and focusing configuration. Photoresists must display the proper sensitivity at the operating wavelength. This technology development often requires optical radiation which is within several nanometers of the actual operating wavelength.
The molecular fluorine laser is a common source of 157 nm optical radiation. However, these lasers suffer from significant drawbacks. For example, they typically are expensive and require constant maintenance due to contamination produced by the laser discharge. The safety precautions required due to hazards stemming from the use, handling, and venting of fluorine gas greatly add to overall system cost and complexity. The radiation produced by these lasers is fixed at certain wavelengths (i.e., the F2 laser is not wavelength tunable) and the beams produced are of low quality. Additionally, the spectral bandwidth of the radiation produced by F2 laser sources is often too large to allow its use in precision metrology applications.
Hence, there is a need for alternate sources of coherent VUV radiation, both as an alternative and as a complement to excimer and F2 lasers. For example, a source capable of producing VUV pulses (especially at 157 nm and/or 121 nm) at high repetition rates would be a viable alternative to the F2 laser for certain applications. Similarly, a lower power VUV system would also be a viable alternative to F2 lasers for certain applications, particularly if the lower power system had other advantages such as lower cost, higher quality beams and/or simpler maintenance. Even in cases where an F2 laser is a good choice for a particular lithography application, the application itself may generate an ancillary demand for alternate sources at similar wavelengths to complement the F2 laser. For example, components used in the lithography application may need to be inspected at the same VUV wavelengths at which they will be used. For various reasons, sources other than F2 lasers may be preferred for these ancillary tasks.
Solid state sources of VUV optical radiation are difficult to create. One reason is that many solid state nonlinear crystals, which have provided solid-state sources of actinic radiation for previous generations of lithographic devices, become opaque at short (sub-200 nm) wavelengths. For this reason, solid-state VUV sources typically consist of one or more solid-state laser sources whose outputs are frequency-mixed in a gas-phase nonlinear medium. Solid-state VUV sources of this sort suffer from low optical conversion efficiencies, necessitating the use of expensive, high-power laser systems.
Incoherent sources, such as lamps, may be used to produce light at 157 nm and 121 nm. However, these sources often are not bright enough.
Thus, there is a need for optical systems which can produce VUV radiation, particularly at wavelengths similar to or exactly equal to the molecular fluorine line and the Lyman-xcex1 hydrogen line, but which additionally overcome some or all of the shortcomings discussed above.
In accordance with the present invention, a nonlinear optical mixer for producing coherent vacuum ultraviolet radiation includes a cell, a pump laser source and a mixing laser source. The cell contains a gas mixture of xenon gas and a phase-matching gas. The pump laser source generates a pump laser beam directed towards the gas mixture. A sum of two photons from the pump laser beam preferentially excites a two-photon transition in the xenon gas. The mixing laser source generates a mixing laser beam also directed towards the gas mixture. Coherent vacuum ultraviolet radiation is produced by the four wave mixing of a photon from the mixing laser beam with the two photons from the pump laser beam. The gas mixture achieves phase matching of the four wave mixing. The coherent vacuum ultraviolet radiation preferably has a wavelength of approximately 157.63 nm (the molecular fluorine laser line) if the coherent vacuum ultraviolet radiation is produced by difference frequency generation or a wavelength of approximately 121.57 nm (the Lyman-xcex1 hydrogen line) if the coherent vacuum ultraviolet radiation is produced by sum frequency generation. It should be noted that although examples will be given with respect to the Lyman-xcex1 line of atomic hydrogen, the examples apply equally to the Lyman-xcex1 lines of atomic deuterium and tritium as well.
In a preferred embodiment, the pump laser beam and the mixing laser beam are loosely-focused, and spatially and temporally overlapping over a region where the four wave mixing occurs. In a preferred embodiment, mercury vapor is used as the phase-matching gas. In another aspect of the invention, the wavelength of the coherent vacuum ultraviolet radiation is tunable, for example by tuning the wavelength of the mixing laser beam.
In further accordance with the invention, a method for producing coherent vacuum ultraviolet radiation includes the following steps. A cell containing a gas mixture of xenon gas and a phase-matching gas is provided. A pump laser beam and a mixing laser beam are simultaneously directed towards the gas mixture. A sum of two photons from the pump laser beam preferentially excites a two-photon transition in the xenon gas. The four-wave mixing of a photon from the mixing laser beam with two photons from the pump laser beam produces the coherent vacuum ultraviolet radiation. The gas mixture achieves phase matching of the four wave mixing.
The present invention is particularly advantageous because it can produce VUV radiation, particularly at wavelengths similar to or exactly equal to the molecular fluorine line and/or the Lyman-xcex1 hydrogen line, and with significantly higher conversion efficiencies than were attainable in the prior art.